The present invention relates to a process for preparing a polyvinyl ester dispersion stabilized with protective colloid and to its use as a solvent-free and plasticizer-free adhesive having increased water resistance of its film for bonding porous substrates.
Dispersion adhesives, which in many cases have a composition based on polyvinyl esters, are frequently formulated in practice with low- or high-boiling solvents. In their function as temporary plasticizers, the solvents serve as aids to film consolidation by lowering the minimum film-forming temperature or the white point of the dispersions. This measure ensures that the adhesives can be processed even at temperatures close to the freezing point of water. Their advantage lies in a minimal adverse influence on the mechanical properties of the adhesive film; a great disadvantage, however, is the emission of solvents into the environment. A list of common solvents suitable for this purpose can be found, for example, in Wood Adhesives, Chemistry and Technology, Volume 1, Marcel Dekker, New York, 1989, on pp. 332/333 and in Volume 2 on pp. 44.
A further function of the solvents is to prevent coagulation of the dispersions when the adhesives are stored at low temperatures. The addition of low boilers such as methanol, ethanol and acetone as antifreeze agents to polyvinyl acetate dispersions is described in Chemical Abstracts 86: 56287s.
Another group of higher-boiling compounds is added to the adhesive dispersions as permanent plasticizers. Dibutyl phthalate and similar compounds, for example, can be placed in this group. Although they remain in the film after drying, they can nevertheless impair the thermal stability of the adhesive bond and its resistance to cold flow. An overview of these compounds can likewise be found in Wood Adhesives, Chemistry and Technology, Volume 1, Marcel Dekker, New York, 1989, on p. 331 and in Volume 2 on p. 43/44.
A solvent-free and plasticizer-free adhesive composition is fundamentally advantageous in terms of environmental and user protection since there is total abandonment both of toxicologically objectionable substances and of substances which pose a threat to the environment or are a source of odor nuisance in the course of use. It can therefore be used in particular in adhesive formulations labeled as "solvent free".
While there are applications which are covered by special water-resistant, polyvinyl ester-based specialty adhesives, based for the most part on chemical crosslinking reactions, such as the construction of wooden windows, for example, in the market even for all-round glues the manufacturer of adhesives base materials is frequently faced not only with the requirement for environmental acceptability but also with that for at least short-term water resistance of the adhesive bonds i.e. without opening of the bonded joints.
DE-A 44 31 343 relates to a solvent- and plasticizer-free adhesive dispersion based on vinyl esters, especially vinyl acetate, which in terms of its performance properties, such as the thermal stability of the bonds, the freeze-thaw stability of the dispersions and the setting rate is comparable with conventional, externally plasticized polyvinyl ester dispersions. It describes a heterogeneous dispersion having a minimum film-forming temperature of 0-7.degree. C. which consists of two homo- and/or copolymeric vinyl esters having a different glass transition temperature (T.sub.g).
It consists firstly of a homo- or copolymer A having a glass transition temperature of more than 20.degree. C., comprising
85-100% by weight of a vinyl ester of carboxylic acids having 1 to 18 carbon atoms (a1), especially vinyl acetate, PA1 0-5% by weight of an .alpha.,.beta.-unsaturated carboxylic acid (a2), PA1 0-5% by weight of an N-functional compound (a3) and PA1 0-5% by weight of a polyethylenically unsaturated monomer (a4), and secondly of a copolymer B having a glass transition temperature of less than 20.degree. C., comprising PA1 45-98% by weight of a vinyl ester of carboxylic acids having 1 to 18 carbon atoms (b1), especially vinyl acetate, PA1 2-50% by weight of an .alpha.-olefin having 1 to 4 carbon atoms (b2), especially ethylene, and PA1 0-5% by weight of a polyethylenically unsaturated monomer (b3). PA1 85-100% by weight of a vinyl ester of carboxylic acids having 1 to 18 carbon atoms (a1), PA1 0-10% by weight of an .alpha.,.beta.-unsaturated carboxylic acid (a2) and/or of an N-functional compound (a3) and PA1 0-5% by weight of a polyethylenically unsaturated monomer (a4), and a copolymer B having a glass transition temperature of below 20.degree. C., comprising PA1 45-98% by weight of a vinyl ester of carboxylic acids having 1 to 18 carbon atoms (b1), PA1 2-50% by weight of an .alpha.-olefin having 1 to 4 carbon atoms (b2) and PA1 0-5% by weight of a polyethylenically unsaturated monomer (b3), the copolymer A being prepared by emulsion polymerization in the presence of a seed emulsion of the copolymer B and the proportion of the polymers A and B in the heterogeneous dispersion being 1:(0.05 to 2), preferably 1:(0.15 to 1).
Said document also relates to a process for preparing the heterogeneous polyvinyl ester dispersions by mixing the dispersion of the homo- or copolymer A with that of the copolymer B, and also to a process for preparing heterogeneous polyvinyl ester dispersion powders by spray-drying the polyvinyl ester dispersions prepared thus in accordance with that invention, and to a process for preparing heterogeneous polyvinyl ester dispersions by redispersing the polyvinyl ester dispersion powders in water.
It is known in principle that dispersions of heterogeneous composition can be prepared not only by mixing dispersions of different polymer composition but also by means of addition polymerizations onto preformed seed particles in multistage emulsion polymerization techniques, with generation in many cases of polymer particles that are of heterogeneous composition as well (see A. Rudin, Macromol. Symp. 92, 53-70 (1995) and patent literature cited therein).
Heterogeneous polymer compositions are known which are produced by seed polymerizations, for preparing adhesives which are based on copolymeric polyvinyl ester dispersions and which contribute therein to improving the adhesive properties.
JP-A 252280/86 (Chemical Abstracts 106: 197587) describes an improved paper adhesive based on polyvinyl acetate with good quick-setting properties and freeze-thaw stability. The dispersion adhesive is prepared by emulsion polymerization of vinyl acetate in the presence of a seed base consisting of an ethylene-vinyl acetate copolymer with ethylene contents in the copolymer of from 10 to 30% by weight. The emulsion polymer is stabilized by polyvinyl alcohol. The adhesive is subsequently formulated with dibutyl phthalate as plasticizer.
JP-A 135575/80 (Chemical Abstracts 96: 86638) relates to seed polymerizations of vinyl acetate onto ethylene-vinyl acetate copolymers with ethylene contents of 10-55% by weight. Polyvinyl alcohol is used as stabilizer. In addition, up to 40% by weight of comonomers can be polymerized together with vinyl acetate. The emulsions find application as quick-setting paper adhesives for water-resistant paper bonds. Substantial amounts of dibutyl phthalate (20 parts by weight per 100 parts by weight of vinyl acetate) are present in the adhesives.
U.S. Pat. No. 3,769,151 describes an adhesive emulsion which is prepared by seed polymerization. The seed base used is a vinyl acetate-ethylene emulsion which is stabilized with partially hydrolyzed polyvinyl alcohol and anionic emulsifier and which has ethylene contents of 5-20% by weight; in the presence of this seed base, a terpolymer of vinyl acetate, ethylene and an unsaturated acid, such as acrylic acid, is prepared which is stabilized with a mixture of partially and fully hydrolyzed polyvinyl alcohol and also emulsifiers. The function of the acid is to improve the adhesion to vinyl films, and, advantageously, to increase the viscosity following uptake of plasticizer, which can also be used in the emulsions.
The adhesive bonds formed from the heterogeneous, solvent- and plasticizer-free polyvinyl ester dispersions of DE-A 44 31 343, although possessing good heat resistance in comparison to externally plasticized dispersions, nevertheless lack adequate short-term cold water resistance which would permit their classification, for example, in durability class D2 of EN 204 (see Comparative Example C1). The dispersions of JP-A 135575/80 apparently possess a certain degree of water resistance but are not free of plasticizer. In U.S. Pat. No. 3,769,151, ethylene is used as comonomer in both polymerization steps. Owing to the reduced glass transition temperature of the principle polymer, the heat resistance of these adhesives must be regarded a priori as low. Plasticizers, such as phthalates, are, moreover, likewise used in the products.